Muscle endurance of the upper and lower extremity

Muscle dysfunction is commonly observed in patients with chronic obstructive
pulmonary disease (COPD). Muscle function is reflected by muscle strength and
muscle endurance. Strength is the capacity of a muscle to develop maximal
force, while endurance is the capacity of a muscle to maintain a certain force.
Loss of one of these components will result in muscle weakness and impaired
muscle performance. At cellular level, lower limb muscles show a reduced
cross-sectional area and fiber type redistribution in COPD patients, resulting
in a reduced proportion of fatigue-resistant slow fibers. In addition, lower
limb muscles showed a reduced oxidative capacity, due to diminished oxidative
enzyme activity. It is unclear whether muscle dysfunction is generalized or if
it mainly affects the lower extremities. The majority of studies on muscle
function assessment in COPD patients focus on lower limb muscles. Particularly
the quadriceps femoris is often assessed, a primary locomotor muscle which is
easily accessible. Nevertheless, the quadriceps femoris may not reflect general
skeletal muscle function, since this is one of muscles that probably are under
used in COPD. Moreover, it is expected that upper limb muscle function is
relatively preserved in COPD patients.
At present, a wide range of test models and protocols is being used for
quantification of muscle endurance in COPD patients. This probably contributes
to divergent results. The methods for muscle endurance quantification can be
divided on basis of muscle activation strategy, involving voluntary effort or
exogenous stimulation. Within voluntary assessment, the exercise conditions
consist of isokinetic, isometric or isotonic contractions of the examined
muscle. Within exogenous muscle stimulation, options comprise stimulation of
the motor nerve, or stimulation of motor end plates. When interested in muscle
endurance, maximal muscle activation is a requirement. A sub-maximal test could
also be appropriate, if it could be confirmed that the same muscle fibres are
activated during each contraction. However, during a sub-maximal muscle
contraction, subjects can vary in contracted muscle fibres to achieve the
requested task. Dyspnea and fatigue, which are frequent features of COPD
patients, can interfere with an exercise test. Consequently, tests involving
maximal voluntary contractions (MVC*s) are likely to obtain sub-maximal test
results when used in COPD patients and might therefore not be appropriate to
quantify muscle function in COPD.
Limitations of voluntary manoeuvres are overcome by non-volitional muscle
activation. The muscle response of electrical stimulation can be measured in an
isometric condition. A benefit of the electrical stimulation technique is that
the generated force accurately reflects maximal strength. Furthermore, obtained
data is independent of patient motivation and central activation factors. The
first option of stimulation is to deliver stimuli to the motor nerve. A
weakness of this procedure is that endurance data is difficult to obtain. It is
a painful technique and constancy of stimulation is hard to preserve because
the stimulation coil can move relative to the nerve. An other disadvantage is
that it can not be used for assessment of the upper limb muscle. That is,
stimulation of the brachial plexus innervates the biceps brachii as well as the
antagonistic triceps brachii. Consequently, the measured external force will
not reflect true biceps brachii strength.
In contrast, stimulation of the motor end plates might be appropriate to
quantify muscle endurance of lower and upper limbs in COPD patients. A great
benefit of stimulation of the motor end plates of a muscle is that the
contraction of the muscle is better tolerated. Up till now, this technique had
only been applied on the quadriceps muscle in COPD patients. In 2007, Swallow
et al. made use of repetitive magnetic stimulation of the intramuscular
branches of the femoral nerve to induce and quantify quadriceps endurance in
healthy subjects and COPD patients. Stimuli were given at 30 Hz, a duty cycle
of 0.4 (2 s on, 3 s off), and for 50 trains. They found a shorter time for
force to fall to 70% of baseline (T70) in the COPD group. In addition, they
concluded that quadriceps endurance, assessed using repetitive muscle
stimulation, can be safely and reproducibly measured in healthy older humans
and in patients with a serious medical condition (COPD). Wüst et al. (2008)
assessed fatigue resistance in smokers and non-smokers, by stimulating the
quadriceps muscle with 30-Hz trains for 2 min (1 s on, 1 s off). The main
finding of this study was a lower skeletal muscle fatigue resistance in
smokers. Furthermore, this type of stimulation seems also applicable to upper
limb muscles.
The effects of electrical muscle stimulation parameters (e.g. amplitude,
frequency and duration) on muscle fatigue of the knee extensors have been
studies in healthy subjects. However, information on the reproducibility of an
endurance test using electrical stimulation for both the lower and upper limbs
is currently lacking. Moreover, reference values on the ratio between lower and
upper limb muscle endurance are not available. Before testing muscle endurance
using electrical stimulation extensively in a clinical population (such as
COPD), the reproducibility of the endurance test needs to be examined in
healthy subjects.

- Examine the reproducibility of the non-volitional endurance test in healthy
subjects. The hypothesis is that the non-volitional endurance test is
reproducible.
- Comparing arm and leg muscle endurance in healthy subjects. It is expected
that arm muscle endurance is lower than leg muscle endurance.
- Comparison of muscle endurance tested at three different stimulation
intensities. It is hypothesised that there is no effect of stimulation
intensity.

All subjects will be recruited at Maastricht University. Participating subjects
have to give written informed consent and are required to visit the movement
laboratory of Maastricht University on five occasions. The subjects will be
instructed to arrive at the movement laboratory in a rested state, they may not
have performed heavy exercise in the preceding 24 hours. During the first
visit, subjects will be familiarized with all equipment and testing procedures
and perform a leg and arm maximal strength test. During the next three visits,
the subjects will perform three leg and three arm endurance tests, on three
different stimulation intensities. During the last visit, one arm and one leg
endurance test will be repeated on the highest intensity of the three preceding
intensities. For the participants, the total duration of the study will be 225
minutes.

Test procedures
Participants will be informed both verbally as in written. Subjects have at
least one day time for reflection before decide if they are willing to
participate. Subject characteristics as age, gender, height, weight and BMI
will be collected. Both the strength and the endurance test will be performed
on a dynamometer (Biodex system 3, Biodex corporation, Shirley, New York,
U.S.), which will be initialized and calibrated according to the manufacturer*s
instructions. In both the upper and lower extremity tests, the right limb will
be measured. All experiments will begin with establishment of the correct
seating position. The correct seating position will be determined during the
first visit and will be replicated during the next visit(s).

Strength test
Subjects will carry out two separate strength tests, one for measurement of
knee extensor strength and one for measurement of arm flexor strength. To this
end, three isometric MVCs will be performed, each lasting 3 s and separated by
120 s rest. A countdown will be given, followed by strong verbal encouragement
to maximize torque. The measured maximal torque (Nm) will be used for
estimation of the percentage MVC that corresponds with the peak torque measured
during the endurance test.

Endurance test
The endurance test will be performed with muscle stimulation. For the upper
extremity endurance test, two rubber electrodes (4 x 6 cm) covered in sponges
will be placed on the biceps brachii motor end plates. For the lower extremity
endurance test, two rubber electrodes (8 x 12 cm) covered in sponges will be
placed on the quadriceps femoris motor end plates. The electrodes will be
secured on the muscle with use of Velcro bandages. Electrical stimulation will
be provided with the TENSMED S84 (Enraf Nonius). The stimulation protocol
consists of 2 s stimulation and 2 s rest. The stimulation frequency will be set
at 30 Hz, with a pulse width of 400 µs.

From the dynamometer output, the peak torque (Nm) and TLIM (number of
contractions until 20, 30 and 40% torque decline) will be calculated.

The total duration of the experiment will be 225 minutes (5 days, 45 minutes
per day). During the first visit, subjects will be asked to perform three
maximal voluntary contractions with their arm flexors and knee extensors on a
dynamometer. During the next three visits, the subjects will perform leg and
arm endurance tests with use of muscle stimulation. These tests will be
implemented on three different intensities. The electrical stimuli (300s: 2s on
2s off, 30Hz) will be delivered trough two rubber electrodes on the motor end
plates of the biceps brachii and of the quadriceps femoris. There are no risks
or benefits associated with this study.